1. Field of the Invention
The present invention generally relates to connecting external devices with host devices and, more particularly, to connecting such devices through interfaces supporting limited power levels.
2. Related Art
As is well known, various types of interfaces may be used to facilitate data communication between host devices and external devices such as hard drives or other peripherals. Certain interfaces may also permit host devices to provide power (e.g., bus power) to external devices through the interface while also facilitating data communication with the external devices.
Such bus powered implementations are particularly desirable for users because they may reduce the number of cables connected between host devices and external devices. For example, a single interface cable may be used to provide bus power connections as well as data communication connections between a host device and an external device. As a result, the bus powered interface may permit a user to avoid having to separately connect the external device to an external power supply.
However, conventional bus powered interfaces typically limit the amount of power that is available through the interface. These limitations can significantly decrease the usefulness of such interfaces. For example, when a compatible external device is connected to a host device by a Universal Serial Bus (USB) 2.0 interface, the host device obtains parameters from the external device in accordance with an enumeration process to register the external device with the host device. During the enumeration process, typically only a low power level (e.g., 100 mA) is provided to the external device. After the external device is registered with the host device, a high power level (e.g., 500 mA) may be provided to operate the external device (e.g., in a fully active state). The USB 3.0 interface (e.g., also referred to as SuperSpeed USB) increases these low and high power levels to 150 mA and 900 mA, respectively.
Unfortunately, the initial low power levels provided during the USB enumeration process are often insufficient to operate external hard drives or other types of devices which require large current draws. For example, in order for a host device to receive parameters from an external hard drive during the enumeration process, it is typically necessary to spin up the hard drive to a normal operating speed in order to read the parameters from the hard drive. Although the high power levels supported by the USB 2.0 and 3.0 interfaces may be sufficient to operate the hard drive after it is registered with the host device, the initial low power levels provided during the enumeration process are often inadequate to reliably power up the hard drives in the manner necessary to read the hard drive.
As a result, users may be forced to power an external hard drive from an external power supply each time the hard drive is connected to the host device. These complications can defeat the purpose of the bus power provided through the interface and frustrate the efforts of manufacturers to provide reliable external devices that may be operated exclusively on bus power. Accordingly, there is a need for an improved approach to interfacing bus powered external devices with host devices.